Design method for WDM optical networks including alternate routes for fault recovery

ABSTRACT

A procedure for designing WDM optical networks provides both working circuits and protection circuits for carrying traffic demands between node pairs. The protection circuits are activated for the purpose of network recovery in case of a fault on a working circuit. Each protection circuit is link-disjoint, and preferably node-disjoint, from its corresponding working circuit. The network is subdivided into logically defined rings, such that each working and protection circuit lies on one or more of the rings. In particular embodiments of the invention, the joint routing of circuits is carried out so as to minimize the total length of the working and protection circuits according to a length that includes a weight for each link. The weight is selected to promote the efficient packing of optical fibers with wavelength channels.

FIELD OF THE INVENTION

This invention relates to methods for designing WDM optical networks,and more particularly to the efficient design of WDM optical networksable to recover from network faults by the re-routing of demands ontoprotection circuits.

ART BACKGROUND

Optical fiber networks are growing in importance as a medium for bothshort-haul and long-haul communications. One category of optical fibernetworks consists of dense wavelength-division multiplexed (DWDM)optical networks. In a DWDM network, each optical fiber carries data ineach of many distinct wavelength channels. With present technology, itis feasible for each fiber to carry eighty or more such channels. At thevarious nodes of the network, wavelength-selective optical elements areused to combine traffic from disparate sources onto a single fiber, andfrom a single fiber, to distribute traffic for disparate destinationsinto distinct fibers.

After a connection between a given source node and a given destinationnode has been established, the connection will typically traverse aroute through zero, one, or plural intermediate nodes and through theoptical fiber links connecting the source, intermediate, and destinationnodes. The number of possible alternative routes for calls between agiven source-destination pair depends upon the number of intermediatenodes, and upon the connectivity of the network; i.e., on the number oflinks converging on or diverging from each node. If the network ishighly connected and the number of intermediate nodes is permitted to belarge, e.g. on the order of ten or more, the number of alternativeroutes for a given call can be quite large.

In particular, flexibility in the routing, as well as the re-routing, ofcalls is afforded by the use of an optical element known as an OpticalCross-Connect (OXC). An OXC, which is typically deployed at anintermediate node, is used to pass transmissions between incoming andoutgoing optical fibers. Through the use of an OXC, any one of aplurality of incoming fibers can be connected to any one of a pluralityof outgoing optical fibers. If the OXC is used in tandem with wavelengthconverters, a given message can not only be switched from fiber tofiber, but also from one wavelength channel to another.

In one known OXC arrangement, for example, incoming transmissions fromone or more fibers are wavelength demultiplexed intosingle-wavelength-channel signals, each on a separate input fiber. Then,wavelength converters are used to place each single-wavelength-channelsignal onto a common working wavelength for the OXC. The OXC thencouples each of the single-wavelength-channel signals, at the workingwavelength, into a respective output fiber. Then, wavelength convertersare used to place each signal on a respective output wavelength channel,which is not necessarily the channel on which that signal arrived.Finally, the output signals are wavelength multiplexed ontocommunication fibers for further transmission through the network.

Although many alternative routes may be available for calls between agiven source node and a given destination node (to be referred tojointly as a “source-destination (S-D) pair”), it is often advantageousto limit the routing to only one or only a few of the possible routes.Route sets are often chosen, for example, on the basis of hop count orlink count; i.e., the total number of links. To keep total accrued delaywithin reasonable limits, it is often desirable to limit route sets tothose routes having the lowest link counts.

The term “network design” as used herein means the layout of thephysical elements of a network, such as the fibers and OXCs, togetherwith the prescribed route sets that avail themselves of those physicalelements in order to meet a pre-specified set of connections or calls. Adesign may be static or dynamic. If dynamic, the design may be subjectto periodic revision. The physical layout might be revised, for example,at periods measured in months or years. The route sets, on the otherhand, could be revised on periods of, e.g., a week, day, hour, or evensmaller unit of time.

A network design is economically efficient if it accommodatessubstantially all of the traffic demand, but does not, on average, havea substantial amount of unused capacity. To promote economic efficiency,it is advantageous when planning an initial or revised network design toconsider the physical layout and the route sets together, in view ofmeasured or anticipated demands. There is still a need forcomputationally tractable design procedures that take such an approachto achieving economic efficiency.

SUMMARY OF THE INVENTION

We have devised a new network design procedure. Our procedure tends toproduce economically efficient network designs. Significantly, thenetworks designed according to our procedure include both workingcircuits and protection circuits for carrying traffic demands betweenS-D pairs of nodes. A “circuit” is a succession of one or morewavelength channels in series, one such channel assigned to each link ofa path from the origin node to the destination node of an S-D pair. EachS-D pair between which there is traffic demand is connected by both aworking circuit and a protection circuit. The protection circuit islink-disjoint, and preferably node-disjoint, from the working circuit.If a working circuit should fail, the protection circuit will beavailable to carry the demand. Thus, the network can recover from afault in the working circuit by switching traffic to the protectioncircuit. Significantly, the working and protection circuits for a givenS-D pair together form a logically defined ring within the network.

Accordingly, the invention in a broad aspect involves a method ofrouting a plurality of demands in a network. Elements of the methodinclude logically subdividing the network into a plurality of rings;assigning to each demand a ring that contains both end nodes of thedemand; and assigning to each demand a working path and a protectionpath along the ring from the source node to the destination node,wherein the protection path is link-disjoint from the working path.

In a second aspect, the invention involves a method of network recovery.That is, upon detection of a failure along a working path of thenetwork, at least one demand previously routed on the defective workingpath is re-routed on a selected protection path. The protection path isselected from a predetermined list. The predetermined list is compiledby applying the routing method discussed above and described below infurther detail as to exemplary embodiments thereof.

Our invention encompasses several different approaches to the problem ofnetwork design. In one approach, which we refer to as DedicatedProtection (DP), the wavelengths assigned to a protection circuit arededicated to the corresponding S-D pair and are not shared with theprotection or working circuits of any other S-D pair. In certain otherapproaches, which we refer to collectively as “Shared Protection onLogical Rings,” the S-D pairs are again grouped into logical rings, butnon-overlapping demands on the same ring are permitted to sharewavelength channels for their protection paths. More precisely, a groupof non-overlapping demands on the same ring can be protected by a singleprotection channel on the ring.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a simplified WDM optical network inwhich working and protection circuits are defined in accordance with thepresent invention in one embodiment.

FIG. 2 is a high-level conceptual block diagram of the invention in oneembodiment.

FIG. 3 is an illustrative, hypothetical demand matrix for the network ofFIG. 1.

FIG. 4 is a flowchart of an exemplary procedure for Joint Shortest-PathRouting according to the invention in one embodiment.

FIG. 5 is a graph of an exemplary cost function for computing linkweights according to the invention in one embodiment.

FIG. 6 is a flowchart of an exemplary procedure for assigningwavelengths to routed wavelength channels according to the invention inone embodiment.

FIG. 7 is a flowchart describing an illustrative implementation of theinvention as practiced for designing a network having a DP, CWR, or SFRarchitecture. The specific choice of architecture is determined by thechoice of cost function. FIG. 7 refers to a cost function, but is notlimited to any particular choice of such function.

FIG. 8 is a schematic representation of a simple network in reference towhich will be described the Intersecting Fiber Ring (IFR) strategy in anillustrative embodiment.

FIGS. 9–11 illustrate successive stages in an illustrative procedure forconstructing an IFR architecture on the network of FIG. 8.

FIG. 12 illustrates the use of a protection path for recovery in the IFRarchitecture of FIGS. 9–11.

DETAILED DESCRIPTION A. Dedicated Protection

An object of the present invention is to design networks in which eachgiven demand has not only a working route from a source node to adestination node, but also a protection route that can be activated incase any link or node fails along the working route. In the DedicatedProtection strategy, which we describe first, a wavelength channel mustbe dedicated solely to the given demand on each link of the protectionroute. Preferably, the protection route should share neither links norintermediate nodes with the working route. If sharing of nodes isunavoidable, then the working and protection routes should still bedisjoint as to links.

In a broad aspect, our design procedure for a network having DedicatedProtection involves mapping traffic demands to selected pairs of routesthat are at least link-disjoint, and preferably also node-disjoint.Thus, the network can recover from any failure of the working path byre-routing on the provisioned protection path without any need tolocalize the failure.

One such mapping is typically carried out for each full (or remainingpartial) unit of demand, as measured in terms of the capacity of onewavelength channel on one fiber. One exemplary such unit is the capacityof one OC-192 line. For brevity, a wavelength channel on a single fiberwill be referred to, below, as a “λ-channel.”

The selection of a route pair is carried out in such a way as to favorroute pairs having a relatively low, and preferably a least, value of alength function. The length function advantageously includes a weightedlink count, in which each link has a respective weight based, at leastin part, on the length of the link, the number of wavelength channelscurrently assigned to that link, and the number of ports or opticaltermination units (OTUs) installed at the two endnodes of the link. Theweight is advantageously selected to have a relatively large value whenthe channels in the fibers of the given link are fully utilized, or whenthe OTUs or the ports in the OXCs at the end nodes of the given link arefully utilized. When the fibers on the given link have some availablechannels or the OXC's at the end nodes have, e.g., some available ports,the weight is advantageously selected to have progressively smallervalues as the number of currently occupied channels or ports increases.The selection of route pairs is advantageously carried out jointly overall of the S-D pairs between which there is demand.

Certain embodiments of the invention include a further step of“coloring” the route pairs; i.e., of assigning specific wavelengthchannels to the respective links of the routed working and protectionpaths, thus to define working and protection circuits. In some cases,wavelength continuity may be imposed; i.e., a given working orprotection circuit must be assigned the same wavelength channel on alllinks. In other cases, the same circuit may be assigned differentwavelength channels on different links. However, the channel assignmentis made in such a way that no two circuits will share the same λ-channelon the same fiber of the same link.

Working and protection routes are illustrated in FIG. 1. Shownschematically in the figure is a simplified network consisting of thenodes numbered 1–8, and the links (not numbered) interconnecting thenodes. Filled symbols (circle, triangle, square) in the figure indicateworking routes, and open symbols indicate protection routes. It will beseen from the figure that S-D pair (1,2) is connected via the workingroute 1-3-2 and via the protection route 1-6-5-2. Similarly, it will beseen that S-D pair (2,4) is connected via the working route 2-7-4 andvia the protection route 2-3-1-4. Finally, it will be seen that S-D pair(3,8) is connected via the working route 3-2-8 and via the protectionroute 3-1-8.

It will be evident from the figure that links can be shared amongvarious working and protection routes. For example, link 1-3 is sharedby the working route for (1,2) and the protection routes for (2,4) and(3,8). In fact, individual optical fibers on a link can be shared inthis manner. However, within a given link, only one route is permittedto occupy a given wavelength channel. Often, the same wavelength channelwill be used on all links of the working and protection routes for agiven demand. However, the wavelength channel may in some cases differbetween the working and protection routes. Moreover, if wavelengthconversion is available, the wavelength channel assigned to a givendemand may vary from one link to another.

Various factors affect the cost of a network design. These factorsinclude the cost to acquire and install optical fiber, and the cost ofoptical elements. The installation cost for a new optical fiber is hereenvisaged as including the cost of upgrading the associated OXCs toprovide interconnection for that fiber. The optical elements includelight sources and detectors at the fiber terminations, and also opticalregenerators if needed. An additional investment in such elements isneeded for each further wavelength channel that is added to a fiber.

Since the cost of an optical fiber grows with its length, economicefficiency generally favors routes having relatively short geographicallengths. However, other factors also affect economic efficiency. Forexample, the cost of acquiring and installing each new fiber isindependent of the number of wavelength channels occupying that fiber.Therefore, a network of relatively few fibers, well packed withwavelength channels, is more cost-effective than a network of manysparsely occupied fibers. In effect, cost efficiency is benefitted whenthe installation cost of each fiber is spread over many wavelengthchannels; i.e., over many demands routed through that fiber.

An automated procedure for routing demands through the network willtypically prefer routes having a least value of some cost function. Forexample, minimum hop routing procedures simply select, from the routesavailable to a given demand, that route having the least hop count. Thecorresponding cost function may be envisaged as a weighted link count inwhich the link weights are all unity. More complicated link weights havealso been described, such as weights that increase as the correspondinglinks become more congested.

We have devised a cost function that uses a new kind of link weight. Incontrast to link weights of the prior art, our link weight is designedto at least partially deter the addition of a new fiber if it ispossible to route a demand on fibers that have already been designatedfor installation. Such deterrence is achieved by including in the costfunction a penalty that is greatest when a new fiber is to be selectedfor routing a demand, and that decreases progressively with the numberof wavelength channels that already occupy the selected fiber. Furtherdetails of our cost function are presented below.

A greater appreciation of our invention may be had by referring to FIG.2. As shown in the figure, the inputs to our network design procedurewill typically include a specification of the nodal locations (box 10)and the internodal distances (box 15). These specifications are useful,inter alia, for computation of fiber lengths which, when multiplied bythe cost per unit length of fiber, give at least a portion of the fibercosts. The connectivity of the network can also be specified through theinternodal distances, by specifying a very large distance, i.e., aneffectively infinite distance, between each pair of unconnected nodes.

It sometimes happens, in the physical installation of optical fibernetworks, that distinct links, i.e., links connecting different nodepairs, share a common underground conduit, or are otherwise emplacedwith a common risk of accidental damage. When there are such sharedrisks, there is a relatively high likelihood that when one link in theshared risk group is damaged, other links in that group will also bedamaged. In such cases, it is desirable to take steps that avoid routingboth the working route and the protection route of a demand through,e.g., the common conduit. One way to deter such common routing ofworking and protection paths is to specify in the input to the designprocedure that all demand routed through the conduit is passing througha common link. Such a common link is “artificial” in the sense that itis part of the description of the computational problem, but need nothave a physical counterpart. In FIG. 2, data specifying the presence ofshared conduits are represented by box 20.

Typical inputs to the network design procedure also include fiber costs(box 25), including the cost per unit length of fiber and the associatedcost of modifying the OXCs. Also included in the typical inputs are thecosts of optical elements (box 30), such as single-wavelength-channellight sources, detectors, and regenerators. Also included are fibercapacities (box 35); i.e., the number of wavelength channels that can becarried per fiber.

The typical inputs to the design procedure also include a matrix ofdemands d_(ij) from the i′th origination node to the j′th destinationnode, in which i and j range over all nodes in the network. (Selfdemands d_(ij) are generally set to zero.) The values assigned to theelements of the demand matrix may be based, for example, on theoreticalpredictions, historical data, or recent measurements. An illustrativedemand matrix for the eight-node network of FIG. 1 is presented in FIG.3. In FIG. 2, the input matrix of demands d_(ij) is represented by box40.

With further reference to FIG. 2, it is seen that a procedure referredto as “Joint Least-Cost-Path Routing” is carried out (block 45) usingdata from at least some of the inputs 10–40. In the procedure of block45, routes are selected for all of the demands specified in input 40.The route selection is “joint” in the sense that the routing of eachdemand is responsive to the routing of the other demands. The routing is“least-cost-path (LCP)” in the sense that of the routes available for agiven demand, that route is preferred that has a least value of a costfunction. Details of an exemplary cost function are presented below.

As indicated at block 50 of FIG. 2, the LCP routing is followed, inexemplary embodiments of the invention, by a procedure for assigning oneor more specific wavelength channels to each of the selected routes. Asfurther indicated in the figure, performance of the design procedure, inexemplary embodiments, leads to a network design (box 55) and acalculated network cost (box 60).

FIG. 4 includes an expanded view of an exemplary Joint LCP Routingprocedure as summarized by block 45 of FIG. 2. At block 70 of FIG. 4,the matrix of demands d_(ij) between each S-D pair ij is obtained. Atblock 75, each demand is routed on a route pair, or “ring,” whichconsists of a working path and a protection path. The working andprotection paths for a given demand are required to be node-disjoint orlink-disjoint; i.e., they must have no common intermediate nodes orcommon links, respectively. (The absense of common intermediate nodesimplies the absence of common links.)

It should be noted that node-disjoint paths can protect against bothnode failure and link failure, whereas paths that are only link-disjointwill not protect against all node failures. Consequently, node-disjointpaths are preferred over merely link-disjoint paths. However, in casesin which suitable node-disjoint paths cannot be found, it is stilluseful to employ link-disjoint paths.

Each pair of link- or node-disjoint paths is selected according to ashortest-path routing criterion applied to the total length of the pair.That is, the total length is the sum of the lengths of the working andprotection paths. In subsequent steps, as explained below, the costfunction used to compute the pair length will depend upon the occupancyof each link of the pair. However, in the initial routing represented byblock 75, the cost function may, for example, assume that no othercircuits are occupying any of the links.

The initial routing results in the reservation of a λ-channel in eachlink along the route-pair assigned to each of the demands. This routingis advantageously performed without imposing a constraint of wavelengthcontinuity. That is, the routing is carried out as though wavelengthconverters were available at every node.

Because the initial link costs can be known α priori, it is convenientto tabulate, in advance, a list of all possible pairs of paths (alsoreferred to herein as “route pairs”) ordered by their corresponding(initial) lengths. The shortest route pairs are then readily selectedfrom the list.

At block 80, all of the demands are arranged in some order, which isadvantageously an arbitrary order. As represented in the figure, thetotal number of demands to be arranged is Δ. Routine 110, which includesblocks 85–100, represents a series of re-routing steps that areperformed in order for each of the Δ demands. Thus, routine 110 containsΔ iterations of blocks 85–100. As indicated at blocks 90 and 100, theweights assigned to each of the links change during each of theseiterations. Thus, the new minimum-length routes may differ from theprevious minimum-length routes. The routing is again made subject to therequirement that working and protection paths be node-disjoint or, ifnode-disjoint paths are not found, that the working and protection pathsbe link-disjoint.

Routine 110 is itself iterated until a convergence criterion is met. Oneappropriate convergence criterion is that in the current iteration ofroutine 110, no demands are rerouted on different rings from theprevious iteration. After convergence has been reached, the coloringprocedure may be carried out, as represented by block 105 of FIG. 4 orblock 50 of FIG. 2.

Before discussing blocks 85–100 of FIG. 4 in detail, it will be helpfulto describe the exemplary cost function y_(ij)(φ_(ij) ^(n)). In ournotation, i and j identify a given link by indexing the nodes at itsinitial and final endpoints, respectively. The superscript n indexeseach successive iteration of routine 110, and φ_(ij) ^(n) represents thenumber of unidirectional λ-channels occupying link ij after the n'thiteration of routine 110.

The exemplary cost function y_(ij)(φ_(ij) ^(n)) is given by theexpression

${{y_{ij}\left( \varphi_{ij}^{n} \right)}\frac{\left( {c_{ij} + {c_{ji}\left\lbrack {1 + f_{ij}^{n} - f_{ji}^{n}} \right\rbrack}^{+}} \right) \cdot \left( {1 - \frac{{\Phi_{ij}^{n}{{mod}W}}\;}{W}} \right)}{\left( {\left( {\varphi_{ij}^{n}{{mod}W}} \right) + 1} \right)^{1}}} + {h_{ij}.}$

In the above expression, W represents the number of λ-channels that canbe carried by a single fiber. Thus, whenever a fiber is filled, i.e.,whenever the occupancy φ_(ij) ^(n) of link ij reaches W, the value ofthe positive factor

$\left( {1 - \frac{\varphi_{ij}^{n}{{mod}W}}{W}} \right)$in the numerator of the above expression decays to 0, and the value ofthe positive factor ((φ_(ij) ^(n)modW)+1)^(t) in the denominator of theabove expression jumps down to 1.

This behavior is illustrated in FIG. 5, which represents the costfunction for an illustrative link whose fibers each carry sixλ-channels. It will be seen that when a fiber reaches full occupation,the cost function decays to a minimum value, which is the cost h_(ij) ofadding a new λ-channel. In terms of physical components, this costtypically includes the cost of the optical transceivers for sending andreceiving signals at the new wavelength.

Initially, and just after each fiber has reached full occupation, thecost function assumes a maximum value, which includes, in addition toh_(ij), the cost c_(ij), of installing one new, unidirectional fiberstrand on link ij in the forward direction i→j and the costc_(ji)[1+ƒ_(ij) ^(n)−ƒ_(ji) ^(n)]⁺ of installing a matching number ofnew, unidirectional fiber strands on the same link in the reversedirection j→i. The term c_(ji) represents a single-fiber cost, the termsƒ_(ij) ^(n) and ƒ_(ji) ^(n) represent, respectively, the number offorward-directed and the number of reverse-directed fibers installedbetween nodes i and j at the n'th iteration. The symbol [. . . ]⁺indicates that if the quantity within the brackets is negative, itshould be replaced by zero. It should be noted that equality betweenƒ_(ij) ^(n) and ƒ_(ji) ^(n) need not be required until the finaliteration of routine 110.

The exponent t that appears in the denominator of the above expressionfor the cost function is an adjustable parameter that controls the decayprofile of the cost function from its maximum to its minimum values. Wehave empirically determined that a fixed t-value of 2.2 gives goodresults. Alternatively, t can be varied by small increments over, e.g.,the range 2–3 while iterating routine 110.

We have described a particular cost function y_(ij)(φ_(ij) ^(n)) forpurposes of illustration, and not for purposes of limitation. Other costfunctions can be used without departing from the spirit and scope of theinvention. However, it is especially advantageous to use a cost functionwhich, like the y_(ij)(φ_(ij) ^(n)) described above, decays toward aminimum value as the occupancy of the fiber that is currently beingpopulated with λ-channels increases from zero toward full occupancy.

Turning again to FIG. 4, we will now describe the steps of routine 110in greater detail. At block 85, the λ-channel previously assigned to thecurrent (i.e., the i'th) demand is cleared on every link along the routepreviously assigned to demand i. At block 90, all of the link weightsare updated in accordance with the new occupancy numbers resulting fromthe removal of demand i. At block 95, demand i is re-routed on the routepair that has the least length according to the latest set of linkweights. At block 100, all of the link weights are again updated.

The coloring procedure, represented by block 105 of FIG. 4, isadvantageously carried out in such a way as to minimize the number ofwavelength converters that will be required, so that component hardwarecosts can be minimized. The coloring procedure is not needed ifwavelength conversion is implicitly available at all nodes.

One exemplary coloring procedure is illustrated in FIG. 6 and describedbelow.

At block 120, all of the routed working circuits and protection circuitsare listed. Multiple listings of the same circuit are permitted if thecircuit represents multiple demands. The working and protection circuitsare optionally constrained to use the same wavelength. Alternatively,they can be permitted to receive different wavelength assignments.

At block 125, the circuit list is sorted by decreasing number of hops.For each link and each fiber, a list of currently available wavelengthchannels is initialized with the maximum number (exemplarily, eighty)unused wavelengths.

At block 130, the first circuit currently in the list is removed fromthe list and the lowest-indexed available wavelength is assigned to alllinks on the circuit. All the fibers used by the circuit are marked asoccupied by the assigned wavelength. If no wavelength is available, thecircuit is marked as requiring a wavelength conversion, as indicated atblock 135.

The procedure of blocks 130–135 is repeated until no circuits remain inthe circuit list. If test 140 indicates that a wavelength was assignedto every circuit, the procedure ends. Otherwise, the procedure continuesat block 145.

At block 145, the least number of wavelength converters is assigned tonodes, such that at least one wavelength converter is assigned to everycircuit marked as requiring a wavelength conversion. Those skilled inthe art will understand that standard techniques, referred to as “greedyheuristics” (which are used, for example, in solutions to the“set-covering problem”) are known for making the required assignment ofwavelength converters.

At block 150, those circuits marked as having wavelength assignments arebroken into sub-circuits such that no sub-circuit has a wavelengthconverter at an intermediate node.

The procedure then returns to block 125, where the list of sub-circuitsfrom block 150 is again sorted by decreasing number of hops. Blocks125–150 are iterated until test 140 indicates that a wavelength wasassigned to every circuit and the procedure ends.

If wavelength continuity is required, the procedure halts at block 140,without proceeding to block 145.

The procedures described above are readily carried out byspecial-purpose or general-purpose digital computational apparatus underthe control of an appropriate hardware or software program.

According to an illustrative scenario for fault recovery in a WDMoptical network having Dedicated Protection, a link failure is initiallyrecognized by each end-node of the affected S-D pair. Then, the affectedend-nodes effectuate switching from the working wavelength orwavelengths to the pertinent dedicated protection wavelength orwavelengths. For each affected wavelength, the equipment at theend-nodes carries out, e.g., an operation analogous to AutomaticProtection Switching (APS) as known in the SONET and SDH hardwaretechnologies. Using such technology, recovery can be effectuated withinseveral tens of milliseconds. Alternatively, both the working circuitand its dedicated protection could simultaneously transmit. In such acase, call recovery could be instituted by continually selecting thebetter of the one or two signals received.

B. Shared Protection on Rings

The recovery of the network from a failure begins with faultrecognition; i.e., when the endnodes of each demand affected by thefailure recognize that a failure has occurred. Then, the switchover toprotection wavelength channels is made. Each demand is mapped to asingle ring; i.e., to a single pair of paths that are link-disjoint, andpreferably node-disjoint. Because each demand is mapped to a singlering, it is not necessary to coordinate the recovery process amongmultiple rings. We believe that with currently available WDM networkcomponents, including optical cross-connects, a single ring cantypically recover within 50–100 milliseconds.

In the preceding discussion, we have described a network designprocedure for use when the wavelength channels assigned to a protectioncircuit are dedicated to the corresponding S-D pair and are not sharedwith the protection or working circuits of any other S-D pair. We referto such a protection scheme as “Dedicated Protection.”

We have devised a class of protection schemes alternative to DedicatedProtection. We refer to one such alternative scheme as “SharedProtection on Logical Rings.” In that scheme, we group node pairs intological DWDM rings, as we did for dedicated protection. However, we nowpermit a group of non-overlapping demands on the same ring to shareprotection wavelength channels. By using optical cross-connects, it isalso possible to share protection wavelength channels across rings.However, the particular approach in which protection wavelength channelsare shared across rings is not preferred, because it increases thecomplexity of network recovery when failures occur.

Shared Protection on Logical Rings can be applied in at least threedifferent ring architectures, which we refer to, respectively, as:Correlated Wavelength Rings (CWR), Separated Fiber Rings (SFR), andInterconnected Fiber Rings (IFR). In the CWR architecture, neighboringrings are permitted to share fibers at the common links, but wavelengthchannels are not shared across rings for active or protection channels.In the SFR architecture, each ring uses only fibers dedicated to thatring. Because there is no sharing of fibers across rings, the SFRarchitecture does not need general-purpose optical cross-connects, andcan instead use the less-expensive 2×2 wavelength-selectivecross-connects. In the IFR architecture, it is not necessary to definethe various rings of the network such that every demand can be routedwithin a single ring. Instead, a demand that spans, e.g., two ringsinterconnected by a common node can be divided into two parts, eachrouted within a respective one of the two interconnected rings. Opticalcross-connects situated at the common nodes are readily used toeffectuate the needed interconnection between rings. The IFRarchitecture has the advantage that it reduces the likelihood ofcreating rings that are relatively large in diameter but are packed withrelatively few of the available wavelength channels.

With reference to FIG. 7, we will now describe an exemplary procedurefor routing and wavelength assignment according to the scheme of SharedProtection on Logical Rings. The procedure of FIG. 7 is applicable tothe CWR architecture, and in certain embodiments it also applies to theDedicated Protection scheme described above.

According to the procedure of FIG. 7, paths and wavelength channels areassigned to demands in a manner that tends to drive down, and preferablyto minimize, a cost function. The cost function takes different forms,according to whether the architecture is Dedicated Protection or CWR.Exemplary cost functions for these architectures will be describedbelow.

Turning now to FIG. 7, the network information including lists of nodesand edges, and the demand list D={d_(ij)} are first obtained, asindicated at block 155. Then, at block 160, a set of shortest rings isobtained and placed in a ring list for each S-D pair between which thereis a demand. Each such set consists of the first R shortest rings thatpass through the source and destination nodes. Such a set is readilycompiled by searching through a list of paths between the S-D pair,ordered by hop count. Alternatively, the k-shortest-cycle algorithmdescribed in the copending and commonly assigned U.S. patent applicationSer. No. 09/537,791, filed Mar. 29, 2000 by G. Liu, is advantageouslyused to compile such a set of R shortest rings for each S-D pair.

In this regard, the length of a ring, also referred to as the cyclelength, may represent either the hop count or the aggregated lengthalong the ring. The hop count is preferred if a port costs more than aunit length of fiber. Otherwise, the aggregated length is preferred.

At block 165, the demand list D is placed in decreasing order by theshortest cycle length of each demand. Also at block 165, all S-D pairsin the ring list are placed in a single, increasing order by hop count.

A ring channel is the combination of a ring, a path, and a wavelengthchannel assigned to a given unit demand. A stream s_(ij) is a singleunit of demand between S-D pair (i,j) to which a ring channel has beenassigned. Block 170 represents the construction of a stream listS={s_(ij)}, as explained below.

To construct the stream list S, the total demand between each S-D pairis decomposed into a set of unit demands, in which one “unit” is thecapacity of one full wavelength channel. All of the unit demands,between all of the S-D pairs, are placed in an ordered list. Each unitdemand is then assigned, in turn, to its shortest ring and to thelowest-indexed wavelength channel. Thus, all demand between a given S-Dpair will be initially assigned to a single ring, and all demand betweenall S-D pairs will be initially assigned to a single wavelength channel.It should be noted that although initial ring assignments have beenmade, the respective streams have not been routed; that is, paths havenot been assigned to the unit demands. The assignment of paths, when itis carried out, involves making a selection for each path between aclockwise and a counterclockwise path on the ring.

Although it is acceptable to randomly order the unit demands in streamlist S, it is currently preferred to order the unit demands according tothe following procedure: First, the S-D pairs between which there isdemand are placed in decreasing order according to the shortest cyclelength of each S-D pair. Then, a first sub-sequence is created bysplitting off one unit demand for each S-D pair, and ordering the unitdemands in the same order as their parent S-D pairs. Then, a secondsub-sequence is created by splitting off a further unit demand, ifavailable, for each S-D pair. The second sub-sequence is ordered thesame way as the first sub-sequence, and then appended after the firstsub-sequence. Further sub-sequences are created and appended in the samemanner, until all of the unit demands belonging to each S-D pair havebeen exhausted.

Loop 235 of FIG. 7 results in the initial assignment or, in lateriterations, the reassignment of a ring channel to each unit demand, inturn. At block 175, the ordered stream list S is obtained. At eachiteration of loop 235, a new stream is obtained from the ordered streamlist S, as indicated at block 180. As indicated at block 205, loop 235exits after the last demand in D has been processed.

Loop 240 of FIG. 7 represents a fine-tuning process in which thepreviously assigned ring channels are replaced by new ring channels insuch a way as to minimize the network cost over all unit demandsjointly. This joint process is carried out in a circulative manner byrepeating all of the iterations of loop 235 until convergence isreached, i.e., until no further re-assignment of a ring channel willfurther reduce the total network cost.

Loop 235 begins at block 180 by getting the next demand d from the listS. If the current iteration is any iteration after the first, a ringchannel has been assigned to d. If such a ring channel has beenassigned, then at block 185 the assigned ring channel is cleared; thatis, the corresponding assigned wavelength channel is deleted from alllinks and rings on which it was routed for demand d. At block 190, alist of wavelength-channel usage for each link, each node, and each ringis updated according to the result of block 185; i.e., the list isupdated to reflect the removal of the ring channel corresponding todemand d.

At block 195, a new ring channel is assigned to d. For assignment to d,a ring channel is selected that minimizes the cost function. The costfunction to be minimized is computed using the usage information asupdated at block 200 of the previous iteration. If the assigned ringchannel requires a new wavelength channel on the assigned ring, then acomplete protection channel is assigned along the ring in the reversedirection. All necessary resources, including links, opticalcross-connect ports, and wavelength channels, are claimed along theassigned path. At block 200, the list of link, node, and ring usageinformation is updated to reflect the new ring channel assignment.

Loop 240 is carried out by repeating loop 235 until convergence isindicated at test block 210.

We will now describe the cost function of block 195 of FIG. 7. The costfunction, which we denote COST(λ, r_(sd) ^(w), r_(sd) ^(p)), dependsupon the index λ of the wavelength to be selected, the routed workingpath r_(sd) ^(w) from source node s to destination node d, and therouted protection path r_(sd) ^(p) from source node s to destinationnode d. (For brevity, the argument of the function COST will be omittedin some of the following discussion.)

Let i and j be the initial and final end nodes of a directed link inr_(sd) ^(w) or r_(sd) ^(w). Let p_(ij)(w_(ij)) be the channel costfunction, to be described below, for the link (i,j). Let w_(ij) be thetotal number of wavelength channels currently being used on the link(i,j). Then COST is given by the sum of p_(ij)(w_(ij)) over all links ofr_(sd) ^(w), plus the sum of p_(ij)(w_(ij)) over all links of r_(sd)^(p), plus the sum, over all initial links i of the working andprotection paths, of a port cost function C_(i).

The port cost function is advantageously devised to disfavor theinefficient use of cross-connects by giving a high cost to the firstport of a new cross-connect, but a low cost to unused ports of existingcross-connects. For example, let each cross-connect have X ports. Thenthe cross-connects are fully utilized at a given node i if the totalnumber N_(i) of ports used at that node is a multiple of X; that is,when N_(i) modX=0. If xcost denotes the cost per cross-connect, then oneexemplary cost function C_(i), devised as described here, is equal toxcost×(1+1/X) if N_(i) modX=0, and is othewise equal to xcost/X.

The channel cost function p_(ij)(w_(ij)) is also advantageously devisedto penalize the underutilization of a resource. In this case, however,the pertinent resource is the wavelength capacity of the link. Thus, itis advantageous to make p_(ij)(w_(ij)) at least partially depend on thenumber w_(ij) of wavelength channels already present on link (i,j)according to a function y_(ij)(w_(ij)) whose shape is similar to thefunction y_(ij) ^(t)(φ_(ij) ^(n)) illustrated in FIG. 5.

Our exemplary channel cost function also depends upon the length l_(ij)of link (i,j), and upon a cost factor G that depends upon the type ofprotection being implemented. G will be described in detail below. Ourexemplary channel cost function is given by:p_(ij)(w_(ij))=l_(ij)y_(ij)(w_(ij))·G.

The function Y_(ij)(w_(ij)), which we refer to as a fiber penaltyfunction, is devised to encourage the use of an existing fiber, ratherthan the addition of a new fiber. As will be explained below, whenshared protection is permitted, the function G is devised to encouragethe use of an already-activated λ-channel rather than the addition of anew λ-channel.

We here define the integer n as the number of fibers already in use on alink. As noted, the integer W stands for the number of wavelengthchannels per fiber.

In the preceding expression, y_(ij)(w_(ij)) decreases as w_(ij)increases from nW, corresponding to an empty fiber, to (n+1) W,corresponding to a fiber fully packed with wavelength channels. Thus, itis cheapest to assign a wavelength already assigned to the ring forprotection, but not yet assigned to a working circuit. The cost penaltygoes up whenever a completely new wavelength is assigned, and goes upeven higher whenever a new fiber is added.

An exemplary form for the fiber penalty function is given by:

${y_{ij}\left( w_{ij} \right)} = {\left\lbrack \frac{1 - \frac{w_{ij}{{mod}W}}{W}}{\left( {{w_{ij}{{mod}W}} + 1} \right)^{n}} \right\rbrack.}$

This function y_(ij)(w_(ij)) jumps to a maximum whenever w_(ij)modW=0;that is, whenever an existing fiber is filled and a new fiber must beadded. Until the existing fiber is filled, however, the functiony_(ij)(w_(ij)) is less for each successive wavelength channel that isadded to the existing fiber.

We will now describe exemplary formulations for the cost factor G which,as noted, depends upon the protection strategy being implemented.

The Dedicated Protection strategy prohibits any sharing of wavelengthchannels for working paths or protection paths. Thus, the working andprotection paths are equivalent, except of course that the working pathswill normally be active and the protection paths inactive. Exemplarily,we assign the same cost factor G to both the working paths and theprotection paths. An exemplary such cost factor for each link (i,j) issimply a constant value of 1.

In the Correlated Wavelength Rings strategy, each circuit is routed andprotected in one of a given set of avaliable rings. For any circuit, thelength of the protection path of such circuit is simply the length ofthe ring in which the circuit has been routed. The protection path thushas a fixed cost that need not be included in the cost factor G.

Each working path must be routed on some ring r, and must be routed theshorter way or the longer way around the ring. Although the shorter wayis generally preferred, routing the longer way around the ring mightsometimes be advantageous for the purpose of balancing the load aroundthe ring.

For routing the shorter way around the ring, an exemplary cost factor Gis determined as follows: if the given wavelength channel is already inuse for protection on ring r but no working path using the givenwavelength channel currently occupies link e_(ij), then the givenwavelength channel is available to link e_(ij) and G takes the value

$\frac{1}{M},$where M is defined as the sum of lengths of all links in the network,divided by the shortest link in the network. More generally, let u(r) bethe number of protection wavelengths assigned to ring r, and letv(r,e_(ij)) be the number of working wavelengths already in use on ringr and link e_(ij). Then: G takes the value 1 if v(r,e_(ij))≧u(r) andlink e_(ij) belongs to ring r; G takes the value

$\frac{1}{M}$if v(r,e_(ij))<u(r) and link e_(ij) belongs to ring r; and G takes aneffectively infinite value if link e_(ij) does not belong to ring r.

For routing the longer way around the ring, the exemplary cost factor Gtakes the values described above, multiplied by the length ratio of ringr to link (i,j).

We will now describe the strategy referred to as Separated Fiber Rings(SFR). In exemplary embodiments of SFR, the stream list is ordered andthe ring list is prepared as described above for CWR. A List ofActivated Fiber Rings (LAFR) is defined and initialized as an emptylist. Let W be the total capacity of a fiber for wavelength channelsEach fiber ring in the LAFR will have 2W wavelength channels, of which Wchannels are working channels and W channels are protection channels.

Each stream is taken, in sequence, from the stream list. If no ringcurrently in the LAFR is available to the current stream, the shortestcycle corresponding to the current stream is placed in the LAFR. In thisregard, a ring is available if some path on the ring has sufficientresources to route the stream. For example, a ring is unavailable to astream if one or both end nodes of the stream lie outside the ring, orif any link of the ring already has its W working channels fullyoccupied.

The current stream is then routed along the shortest available ring inthe LAFR. This stream-routing procedure is repeated until all streamshave been routed.

Then, a fine tuning procedure is carried out. According to suchprocedure, each stream is again selected in sequence. The current routesof the selected stream are cleared from the fiber ring currentlyoccupied by the selected stream. Then the stream is re-routed along theshortest available path among all of the rings in the LAFR. This finetuning procedure is repeated until no stream can find a shorter routethan its current route. In this context, the length of a given routewill typically be an appropriately weighted length or hop count.

The strategy that we refer to as Interconnected Fiber Rings (IFR) willnow be described with reference to FIGS. 8–12. For simplicity ofpresentation, it will be assumed that wavelength conversion is availableat all nodes of the network. In such a case, the assignment ofwavelength channels to routed demands is straightforward and does notneed further discussion.

Shown in FIG. 8 is a network comprising nodes 300 and links 310. Thenodes are numbered in the figure from 1 to 8. Initially, each totaldemand (i.e., not necessarily a unit demand) is routed along the shorterpath of its shortest cycle. Length in this regard may be defined withreference to an appropriate link weighting function. After all of thedemands have been routed along their shortest paths, a sufficient numberof fibers is acquired on each link to support all of the demand routedon that link. The result is a working fiber network as illustrated,e.g., in FIG. 9. It will be noted that in the example of FIG. 9,different numbers of working fibers 315 are acquired by different links.That is, links 3-4, 4-5, and 5-6 have each acquired a single fiber,links 1-2, 1-8, 2-6, and; 5-7 have each acquired two fibers, and links2-3, 3-6, 6-8, and 7-8 have each acquired three fibers.

A set of protection fibers is acquired in addition to the workingfibers. The protection fibers are acquired in rings, such as rings 320,325, and 330 of FIG. 10. Significantly, sufficient protection fibers areacquired to cover all of the working fibers. In this regard, the set ofprotection rings covers the working fibers if, given a cut across anylink of the network, the total number of protection fibers passingthrough the cut is at least the number of working fibers passing throughthe cut. The concept of covering is illustrated in FIG. 11, in which itis seen that there is a total of three working fibers on link 2-3, and atotal of four protection fibers, distributed over two distinctprotection rings, on the same link.

It is advantageous for the covering set of protection rings to be aminimal set, in the sense that the total fiber length over allprotection rings is minimized.

One useful method for generating a covering set of protection rings isprovided by the Joint LCP Routing procedure of FIG. 4. When such aprocedure is applied in the IFR context, each fiber in the working fibernetwork of, e.g., FIG. 9 is treated as an individual, single-hop demand.

After a covering set of protection rings has been defined, each workingfiber on a given link is assigned to a particular one of the protectionrings.

A recovery scheme is illustrated in FIG. 5. The demand 3-7 is routed onthe working path 3-6, 6-8, 8-7. In case of a failure on the workingpath, the demand is routed over two covering rings, on the path 3-2,2-6, 6-5, 5-7.

1. A method of routing a plurality of demands in an optical network thatcomprises nodes interconnected by links comprising optical fibers, eachdemand having two end nodes, the method comprising: a) logicallysubdividing the network into a plurality of rings, wherein each ring isformed by two link-disjoint paths between a pair of nodes; b) to each ofthe demands, assigning a ring that contains both of the pertinent endnodes; c) to each of the demands, assigning two mutually link-disjointpaths on the ring from one end node to the other, wherein one said pathis a working path and the other said path is a protection path; and d)assigning at least one wavelength channel to each working path and toeach protection path, resulting in a working wavelength channel on theworking path and a protection wavelength channel on the protection path,wherein: the path and wavelength-channel assignments are carried out soas to drive down a cost function; the cost function includes, for eachlink, a cost component for placing a further wavelength channel on suchlink; and said cost component is selected to decrease as the number ofalready-placed wavelength channels increases, but to jump to a highestvalue when the number of already-placed wavelength channels reaches thefull capacity of one optical fiber.
 2. The method of claim 1, whereineach of the protection paths is node-disjoint from its correspondingworking path.
 3. The method of claim 1, further comprising, for at leastone pair of end nodes, subdividing a total demand between said end nodesinto a plurality of unit demands, and wherein the assigning of workingpaths and protection paths is performed on the unit demands.
 4. Themethod of claim 3, wherein one unit of demand is equivalent to thebandwidth capacity of one wavelength channel on an optical fiber.
 5. Themethod of claim 1, wherein the assignment of wavelength channels iscarried out such that no two demands have the same working wavelengthchannel or protection wavelength channel.
 6. The method of claim 1,wherein the cost function is further determined by the occupancy ofports or optical termination units at nodes of the network.
 7. Themethod of claim 1, wherein the cost function further includes a costcomponent for placing wavelength ports at end nodes of the link, and thecost component is selected to decrease as the number of already-placedwavelength ports increases, but to jump to a highest value when thenumber of already-placed wavelength ports reaches the full capacity ofone optical cross-connect.
 8. The method of claim 1, wherein the pathand wavelength-channel assignments are carried out such that theassignments to the respective demands jointly drive down the costfunction.
 9. The method of claim 1, wherein rings having a common linkare permitted to share optical fibers on such common link.
 10. Themethod of claim 1, wherein: the assignment of wavelength channels iscarried out such that on a given ring, the protection paths of two ormore demands are permitted to share the same wavelength channel if therespective working paths of said demands have no common link on thegiven ring.
 11. The method of claim 10, wherein: two or more ringshaving a common link are permitted to share optical fibers on such acommon link; and each wavelength channel on such a shared optical fiberbelongs exclusively to only one of the sharing rings.
 12. The method ofclaim 10, wherein each optical fiber on a given link of a ring isallocated exclusively to one ring.